It depends on the instructor. I feel like spinning classes are great for the cardiovascular, provided you have a strict training schedule, a heart rate monitor, and a good instructor. However, a lot of the time, you have some really bad ones. I think the best thing you could do is audit classes at a club BEFORE you pay and sign on to something you wish you never walked into.
BTW, there's a woman named Iona Passik who's out of the NY Sports Club in Manhattan. I think she's one of the best instructors I've ever taken classes from- she's very sensible, she knows her stuff, she has great music, and she creates these great visualizations that really get you in THE ZONE. I've walked away from her class in the shivers, it was sooooooo goooooood. She's a master presenter for Mad Dogg, and the presenters in the USA aren't that great, but she is heads above the rest of them.